1. Field of the Invention
The present invention relates to an optical fibre telecommunications cable with reduced polarization mode dispersion, and to the corresponding method of production. The present invention also relates to an optical fibre suitable for use in an optical fibre telecommunications cable, and to the corresponding method of production.
2. Description of the Related Art
As is known from “Fiber-Optic Communication Systems”, Govind Agrawal, John Wiley and Sons, Inc., Second edition, a single mode optical fibre with a perfectly symmetrical core and of uniform diameter permits the transmission of two degenerate orthogonal modes (typically known as TE and TM), having the same value of mode index (or effective index) n, defined as n=β/k0, where β is the propagation constant and k0 is the free space wave number.
The cylindrical symmetry of the core can be interrupted by variations in the shape of the core, originating for example during the process of drawing the fibre.
The process of drawing an optical fibre is usually carried out by means of an appropriate apparatus known as a “drawing tower”, from a suitably prepared glass preform. In practice, the preform is placed in a vertical position in a furnace where a lower portion of the preform is heated to a temperature above the softening point. The material flowing from the preform is drawn downwards at a controlled rate in such a way as to produce a filamentary element which forms the optical fibre. In this process, variations of shape in the core of the fibre can be due, for example, to structural and geometrical defects in the preform, or to undesired variations of the operating conditions of the process.
Owing to the aforesaid variations of shape of the fibre core, the optical properties along two orthogonal axes x and y in a section of the core become different, and the fibre acquires birefringence. If nx and ny are the mode indices for the modes polarized along the two Cartesian axes x and y, the birefringence is given by B=|nx−ny |.
When an optical signal passes through a birefringent fibre, a phenomenon known as the “polarization mode dispersion” (PMD) occurs, and this causes the two modes with orthogonal polarization to be propagated at different phase and group velocities. In the case of pulsed signals, the PMD typically causes a spreading of the pulses. This is because, if an incoming pulse excites both polarization components, the two components are dispersed along the fibre as a result of their different group velocities, and the pulse leaving the fibre becomes wider.
The phenomenon of PMD usually gives rise to a limitation of the signal transmission bandwidth and, consequently, a degradation of the performance of the optical fibres. This phenomenon is therefore undesired in optical fibre telecommunications systems, especially in long-distance systems, in which it is necessary to minimize all kinds of attenuation or dispersion of the signals in order to provide high performance in transmission and reception.
Typically, the phenomenon of PMD increases in a linear way with the length of the fibre. However, longitudinal imperfections and irregularities in the fibre produce an exchange of power between the two polarization modes, giving rise to the phenomenon of “mode couplings”. Owing to this phenomenon, the delay between the two polarization modes varies, along the axis of the fibre, in a slower way, particularly in proportion to the square root of the distance travelled.
The mode coupling can also be imposed deliberately by suitable methods, for example by the method of “spinning” the fibre during drawing, which consists in imparting to the fibre a predetermined spin about its axis, as described for example in U.S. Pat. No. 5,298,047 in the name of AT&T Bell Laboratories. The spinning method makes it possible to reduce the PMD of a single mode fibre to less than 0.1 ps/km1/2. Values of this order of magnitude are usually requested from fibre manufacturers by those constructing telecommunications systems. The acceptable values of PMD can, however, vary according to other system parameters, particularly the transmission rate and the length of the links.
The spinning method can also be used in the process of producing doped fibres for use in optical amplifiers, as described in U.S. Pat. No. 5,704,960 in the name of Corning, but in order to reduce two phenomena present in this type of fibre, known as “Polarization Hole-Burning” (PHB) and “Polarization-Dependent Gain” (PDG). Also according to U.S. Pat. No. 5,704,960, in order to clearly define the requisite spin and the reversing length of the fibre (in other words, the distance between two reverses of the direction of spin present in an alternating spin), the fibre can be fabricated with a finite birefringence by ellipticity or stress. This finite birefringence must be capable of predominating over the unintentional birefringence introduced during the fabrication process.
The applicant observes that, although U.S. Pat. No. 5,704,960 does not specify quantitative values of the finite birefringence to be applied, it is necessary to have a high ellipticity of the fibre in order to obtain a birefringence capable of predominating over the unintentional birefringence (and therefore considerably larger than the unintentional birefringence). In particular, the applicant considers that the requisite ellipticity is such that there is an increased complexity and cost of the processes of producing the fibre and assembling the telecommunications system. This is because, in the production process, if the fibre has to be made with a high ellipticity, the control of the optical specifications of the fibre is inevitably less precise. This is due to the fact that, in the fibre production process, which is a complicated process because it has several degrees of freedom, it is also necessary to allow for the high ellipticity of the fibre and its effect on the other process parameters. Moreover, during the assembly of the telecommunications system, if the ellipticity of the fibre is high, the correct joining of the fibres becomes difficult and requires complex and expensive equipment.
Usually, the optical fibres for signal transmission are cabled after the drawing process. Typically, the cabling process comprises the production, by an extrusion process, of a polymer body containing the optical fibres, followed by the application of external protective and reinforcing layers and elements.
In cables known as “tight” cables, the optical fibres are incorporated directly, together with a central supporting element, into the polymer body. In cables known as “loose”, a plurality of tubular bodies of polymer material, arranged around a central element and each containing a plurality of optical fibres, are placed within an outer tubular body which is also made from polymer material.
The aforesaid cabling operations comprise, particularly for “tight” cables, the application of non-uniform forces to the optical fibres, and consequently the generation of contributions to birefringence due to lateral pressure or elastic spinning of the fibres.
In greater detail, the applicant has observed that, during the different steps of the cabling process, radial stresses are induced within the cable, which stresses extend throughout the length of the cable and, particularly in “tight” cables, can cause lateral deformations of the optical fibres. For example, at the end of the extrusion process, tensions can be “frozen” into the polymer body, giving rise to a state of continuous radial stress. Similar effects are caused by the contraction (“shrinkage”) of the extruded material during the cooling which follows the extrusion process.
Other stress contributions can be created by the radial stress generated in subsequent steps of processing, as in the step of pressurizing the polymer body containing the fibres or during the normal application of a polyethylene sheath around the polymer body.
Although these stress states are usually distributed uniformly in the angular direction, there is a non-uniform deformation of the optical fibres (and therefore an increase in birefringence), since the optical fibres are not positioned along the axis of the cable.
Moreover, the birefringence can increase as a result of the ovalization of the outer reinforcing and protective coverings. In this case, the stresses act radially but are not uniformly distributed in the angular direction, since they act predominantly on the lateral portion of the cable in which the deformations are concentrated. These stresses can act on the optical fibres in a continuous or periodic way, depending on whether the fibres are placed parallel to each other or are wound (in a cylindrical helix) around the central element. In the latter case, the effect of the stresses can depend on the periodicity of the winding of the optical fibres.
The variations in shape to which the cores of the optical fibres are subjected by the cabling process therefore contribute to influence the performance of the fibres in terms of PMD.
The applicant has observed that, although the spinning method applied in the course of drawing is effective in reducing the PMD caused by intrinsic defects generated in the course of drawing, it is not so effective in reducing the PMD caused by the effects of cabling. This :is primarily due to the fact that the contributions of birefringence due to cabling are usually deterministic, since they are not orientated in a random way, and therefore the PMD of a cabled fibre is generally difficult to predict. To overcome this disadvantage, particular attention is paid in the prior art to the design and construction of the cable, in order to cause the smallest possible stress on the fibres.
U.S. Pat. No. 5,867,616 in the name of Corning Incorporated proposes a method for controlling the PMD in a way which is fully independent of the processes carried out after the drawing process, for example the process of “buffering” (in other words the application of a secondary protective covering, which can be “loose” or “tight”) and the cabling process. This patent proposes that a coupling of the polarization modes within the fibre be produced, by causing periodic variations of ellipticity of the core, of concentricity between the core and the cladding, or of residual stress in the fibre. These variations are produced in such a way that their planes of symmetry are orthogonal to each other in adjacent segments of fibre which are essentially of the same length. The resulting net birefringence is therefore zero.
The applicant observes that, in U.S. Pat. No. 5,867,616, the deliberately induced birefringence has to be of sufficient extent that it can eliminate, by itself, the effects of birefringence due to the cabling and “buffering” processes. The applicant has calculated that the variation of the refractive index of birefringence due to the cabling process is usually less than 3×10−7, and therefore notes that a deliberately introduced variation of the refractive index such as that proposed in U.S. Pat. No. 5,867,616, in other words one of at least 1×10−6, is much greater than the variation of the refractive index of birefringence arising from the cable production process. The applicant considers that the modifications required to introduce this level of birefringence can give rise to greater complexity or cost in the fibre production process and at the stage of assembly of the telecommunications system, for the reasons discussed previously with reference to U.S. Pat. No. 5,704,960.
The applicant has addressed the problem of providing a method which reduces the aforesaid disadvantages due to the cabling process, without giving rise to significant complication or increases in cost in the fibre and cable fabrication process or at the stage of assembly of the telecommunications system.